In many situations there is a need for determining, quickly and reliably, the moisture content of a material in the form of particles. Such a material can be a component in a mixture having particular mixing proportions, which is to be dried to attain a certain maximum moisture content, etc. A few examples will be stated below in order to illustrate the range of application of the invention.
When road surfacings are produced, the main ingredient being asphalt, stones and gravel are used as aggragates, which are added to the asphalt according to various formulae. Before the aggregate material is added to the asphalt material, it is dried and heated to a selected temperature, which is quite important for the quality and the hardness of the end product. The furnace is heated with oil burners, which are adjusted depending on the temperature variations, which are measured along the furnace. This control technique means that the temperature of materials, which have already left the furnace, is gauged. However, it is desirable to be able instead to gauge the moisture content in and the temperature of the fed material in order to adjust the burner in view of this and be able to reduce the oil consumption.
When dry mortar is produced, the conditions are similar to the conditions, when road surfacing is produced, but there is a difference, that the temperature after the drying of the aggregate material must be as low as possible. The object is to consume energy to dry the aggregate material but not more than that. At temperatures above what is needed energy is lost to the environment. Also, an elevated posttemperature causes problems for the transport equipment after the drying process.
When concrete is produced, it is important that the amount of water in the aggregate material is determined quickly and carefully, since this water greatly influences the amount of water to be added afterwards. The water in the aggregate material may vary from a few per cent, when using a dried aggregate material, to as high as 30-40% of the total amount of water in the completed concrete mixture. The ratio between cement and water in the completed concrete must be kept within selected limits in order to attain the desired quality. In case there is uncertainty as to the amount of water, often too much cement is added. Consequently, a careful determination of the amount of water is of great economic importance when producing concrete. Also, a wrong amount of water impairs the concrete and sometimes it may render it useless, resulting in rejections.
In the stone-working industry it has been found that the moisture content influences the milling process, when stone powder of various types is produced. Consequently, the artisan wants to be able to vary the mutual adjustment of the grinding bodies as the moisture content varies, partly to obtain a uniform quality and partly to reduce the consumption of wear metals.
In the cellulose industry quick lime is used in some processes and it is recycled by burning it in lime sludge furnaces. The lime is charged into the furnaces in the form of dehydrated sludge, the moisture content of which varies due to the condition of the filters, the particle size of the lime etc. Large amounts of energy are needed for the burning in the lime sludge furnaces and in order to be able to adjust the burners and consequently to lower the amount of fuel it is advantageous to thoroughly be aware of the moisture content of the charged material.
When chips, peat, coal etc. are burned the moisture content of the fuel is an important factor of the combustion in the boiler. An apporpriate air supply in relation to the heat value and the moisture content results in a superior economy and an exceedingly small amount of air-contaminants. Customarily the boiler is adjusted by i.e. gauging the flue gas temperature. It is quite obvious that it may already have been damaged, when high temperatures are recorded, and it would be advantageous to be able to gauge the moisture content of the fuel, in order to control the boiler in advance.
In order to solve the above stated and similar problems it is a must to be able to quickly and carefully determine the moisture content of the material. When producing concrete today prevailingly a sample method is used to determine the moisture content of the aggregate material. This means that sampling is performed with certain intervals, the moist material is weighed, dried and weighed again, in order to obtain a measure of the moisture content. Obviously, this method is slow and tedious and also not reliable, since the aggregate flow through the plant is so large, that when the sampling outcome has been obtained, the aggregate, which is to be fed to the mixer, may be an entirely different aggregate from the one when the sampling was performed. It is also obvious that this sampling method can not be used in those instances, when speed is an important factor, e.g. when adjusting boiler plants etc.
Hence, also considerably quicker gauging methods have been developed. In one method of this kind radroactive isotopes are used in order to count the number of water molecules within a sphere of a container, in which the moist material is kept. A drawback of this method is that it partly gauges a material, which due to the flow conditions in a silo is not used but remains there, and partly also gauges the water of crystallization and that amount of water, which is bound in the granules due to capillary effects and consequently does not influence the concrete production.
Another suggested method is based on the idea of gauging fluctuations in electric conductivity for different moisture contents. However, this method is unreliable due to the fact, that the inherent electric characteristics of the material strongly influences the measurement result.
It is also known how to use the heating capacity of the material in order to determine the moisture content. Such a method and appropriate equipment are described in DE-OS No. 31 47 195. In accordance with the method, which is described in this patent specification, a measuring body, which is inserted in a stationary material, e.g. leather or another fibrous material, is heated, first to a certain selected preheating temperature of between 40.degree. and 90.degree. C. Subsequently the measuring body is heated to a certain selected temperature of between 90.degree. and 140.degree. C. in order to dry the enveloping material. The drying time, which is dependent on the moisture content, is recorded. By comparing the recorded drying time and earlier recorded calibration times, a measure of the moisture content is obtained. However, also this method is characterized by some definite restrictions. The most serious restriction is that the method can not be used for flowing materials.